1. Field of the Invention
The present invention relates to a method of forming an insulating film an inner surface of a through-hole provided in a conductive substrate or a semiconductive substrate by using an electrodeposition coating. The present invention further relates to a semiconductor apparatus in which a conductive film is further formed on an inner surface of the thus-formed insulating film such that conduction can be established between its front surface and its rear surface.
2. Description of the Related Art
Electrodeposition coating techniques have been conventionally used for coating components having complicated shapes, such as parts of motor cars and electronic apparatuses or equipment. Generally, thicknesses of the electrodeposited films when hardened are in a range from 20 microns to 100 microns. During electrodeposition coating, the thickness of a wet-coated film is normally reduced by 80% to 90% when the film is hardened. At the time of hardening and shrinkage of the wet-coated film, thermal flow occurs from its thick portion toward its thin portion. Accordingly, a very flat coated film can be obtained.
In the event that an edge portion is present in the coated component, the thickness of a wet-coated film at the edge portion prior to hardening is likely to be thicker than that at a flat portion since current is concentrated at the edge portion. In the wet-coated film at the edge portion, thermal flow toward the flat portion occurs at the time of hardening. It is accordingly likely that an underlying substrate at the edge portion is exposed, and remains uncoated, since an excessive thermal flow occurs at the edge portion.
Therefore, it is known to use an electrodeposition coating material containing a fluidity adjusting agent to control the thermal flow. The fluidity adjusting agent is a material for controlling the thermal flow. An electrodeposition coating material can be obtained by adding a filler with its main component silicon, silica and a polymer, such as fatty acid amide wax, in the electrodeposition coating material in amounts from about 1% to about 30%. However, when electrodeposition coating is carried out using a fluidity adjusting agent, thermal flow of the electrodeposition film becomes difficult to occur at the more interior, flat portion due to the polymer of the fluidity adjusting agent. The edge portion, however, is appropriately covered with a film having a certain thickness at the time of hardening. Accordingly, since unwanted stress is applied to the more interior, flat portion, it is likely that the surface of the coated film undulates and the flat portion is less uniformly flat.
Japanese Patent Application Laid-Open No. 6(1994)-57496 discloses a two-step electrodeposition of a two-coat and two-bake type. In the two-step electrodeposition, an electrodeposition coating material with a good thermal fluidity is coated onto a surface by a first electrodeposition coating, and hardened. The flatness of a flat portion other than an edge portion is acquired by the first electrodeposition coating. At the edge portion, the surface is exposed, and it remains uncoated. In a second electrodeposition coating, an electrodeposition coating material containing a fluidity adjusting agent for controlling thermal fluidity is then coated, and hardened. Since the second electrodeposition coating is selectively conducted only on a surface of a conductive material, the coating can be selectively carried out on the edge portion that is not yet coated during the first electrodeposition coating. Thus, the two-step electrodeposition of the two-coat and two-bake type can achieve an electrodeposition coating in which the entire surface of the edge portion and the flat portion is fully covered, while the flatness of the flat portion is secured.
In recent years, electrodeposition techniques are used not only for coating on external cover components, such as components for motor cars and electronic apparatuses, but also for formation of an insulating film that is necessary when a conductive pattern is formed on a conductive substrate or a semiconductor substrate. Particularly, in the event that a through-hole provided in the conductive or semiconductive substrate is used to establish conduction between its front surface and its rear surface, an insulating film is initially formed on an inner-side surface of the through-hole, and a conductive film is further formed on an inner surface of the insulating film. The electrodeposition technique can be employed for the formation of the insulating film.
The inner diameter of the through-hole decreases to a very small magnitude, for example, to a range from 50 microns to 150 microns, as the density of the conductive pattern increases. It is accordingly required for the insulating film formed on the inner surface of the through-hole that its thickness be in a range between about 2 microns and about 20 microns, and that it has excellent flatness. The reason for this is as follows. Electrical leakage to the insulated conductive material occurs if the flatness is not uniform and undulations form and the thickness of the insulating film decreases to about less than 2 microns. Therefore, its insulation properties become poor, and the function of the insulating film is likely to be nullified. Further, if the thickness of the insulating film at the time of hardening increases to about more than 20 microns, it becomes very difficult to form the conductive film on the inner surface of the insulating film.
The thickness of the insulating pattern used in the conductive or semiconductor substrate is normally in a range between 2 microns and 20 microns, and this range of the required thickness is significantly different from a range of the thickness of the coated film used in the conventional components of motor cars, electronic apparatuses or the like. Hence, it is quite difficult to form an insulating film having a thickness from 2 microns to 20 microns on the inner surface of a though-hole whose inner diameter is in a range between 50 microns and 150 microns.
In the event that a coated film is formed by a single electrodeposition of an ordinary electrodeposition coating material without any fluidity adjusting agent, an underlayer at the edge portion in close proximity to an opening portion of the through-hole is liable to be exposed since an excessive thermal flow occurs at the edge portion as discussed above.
Where a coated film is formed by a single electrodeposition of an electrodeposition coating material containing the above-discussed fluidity adjusting agent, formation of a wet-coated film having a thickness of 60 microns is needed for formation of an electrodeposition film having a thickness of 20 microns. If the wet-coated film having a thickness of 60 microns is formed on the inner surface of the through-hole, a wet-coated film with a thickness of 80 microns is likely to be formed at the opening portion of the through-hole. The opening portion is therefore enclosed by the film, and there is a great possibility that the closure of the through-hole occurs. If closure at the opening portion of the through-hole occurs, degradation of conduction appears since it becomes impossible for the conductive film formed on the inner surface of the insulating film to establish conduction between the front surface and the rear surface of the through-hole.
Further, the coated film is likely to undulate and lose flatness since unwanted stress is applied to the flat portion at the time of hardening of the wet-coated film, as discussed above. If the surface of the insulating film becomes more irregular and the film thickness is decreased to about less than 2 microns, electrical leakage to the insulated conductive material occurs and its insulation is degraded.
Also, in the case of the formation of the coated film by the two-step electrodeposition of the two-coat and two-bake type disclosed in the above-discussed Japanese Patent Application Laid-Open No. 6(1994)-57496, there is a great possibility that the opening portion is choked and closure of the through-hole occurs, similar to the case of the formation of the coated film by a single electrodeposition of the electrodeposition coating material containing the fluidity adjusting agent.
Furthermore, in the case of the two-step electrodeposition, the through-hole is very likely to be choked with the first coated film. The reason for this is as follows. Since the first wet-coated film contains no fluidity adjusting agent, thermal flow of the material occurs due to its surface tension at the time of hardening, and the material is caused to flow from a thick portion of the wet-coated film at the opening portion into a thin portion of the wet-coated film on the inner surface of the through-hole. Accordingly, in the event that the inner diameter of the through-hole is very small; for example, about 50 microns to 150 microns, closure is created in the through-hole. If closures are created in the through-hole and at the opening portion of the through-hole, degradation of the conduction occurs since it becomes impossible for the conductive film formed on the inner surface of the insulating film to establish conduction between the front surface and the rear surface of the through-hole.